The high circuit densities encountered in modern radios make it difficult for radio manufacturers to meet the spacing specifications (intrinsic safety spacing requirements) laid down by intrinsic safety testing agencies (e.g., Underwriters' Laboratories (UL)-United States, Factory Mutual (FM)- United States, BASEEFA- United Kingdom Testing Agency, CENTALEC-Federal Republic of Germany, etc.)for achieving intrinsic safety approval ratings. Both component-to-component and layer-to-layer spacing distances, have to be well above the specified minimum spacing distances, (typically have to be greater than 0.5 millimeter) in order to meet the spacing constraints placed on radio manufacturers by intrinsic safety testing agencies.
The spacing requirements imposed by the testing agencies (in order to meet intrinsic safety approvals) are seldom achieved by radio manufacturers, due to the great amount of integration modern radios are required to have in order to achieve their small size and at the same time still retain all of their numerous user features. Not meeting the minimum spacing requirements causes the testing agencies to consider the whole radio as a "mass fault." This allows the testing agency to sum all capacitances and/or inductances and place them as if they are connected to the test point (node) they are testing in the radio. They can also assume the worst case voltage and current levels, and also assume the greatest power output capable of being achieved by the radio, when running any approval tests. This in turn forces the radio manufacturers to limit current and voltage levels in the radio to levels considered safe enough to meet intrinsic safety requirements by intrinsic safety testing agencies.
Typical maximum values imposed by the agencies, once a radio is considered a mass fault, are in the order of a maximum operating voltage of six volts and no more than a maximum of 250 milliamps current output at the battery. For more stringent approval ratings, not more than 50 milliamps of current can be sourced by the battery. The main problem with limiting a radio to these low current and voltage values, is that these values limit the total amount of power output the radio would be capable of delivering. These very restrictive value settings would not allow high powered radios to meet intrinsic safety levels.
In FIG. 1, there is shown a block diagram of a prior art portable radio with battery 100. Battery 102 consists of a plurality of rechargeable nickel-cadmium battery cells connected in series, forming a rechargeable battery pack 108. Coupled to the positive terminal 114 of battery pack 108, are a redundant set of current and voltage limiter circuits 104. The output 112 of the voltage/current limiter circuits, provides a regulated output for portable radio 106. A second output 110 is provided by battery 102, which provides the ground potential connection to radio 106. The major problem with the prior art radio and battery 100 shown in FIG. 1, is that radio 106 would still be limited to the low voltage and current levels (e.g., 6 volts, 250 milliamps) required by the testing agency in the case the spacing requirements in the radio 106 are not being meet.
A need thus exists in the art for a way of achieving higher levels of intrinsic safety standards in portable electronic devices without having to meet the spacing limitations placed on the devices by the testing agencies, or without having to limit the amount of voltage and current capable of being supplied by the battery to the device in order to meet the requirement.